Recent developments in catalyst technology have provided processes for the conversion of hydrocarbon feeds in fluidized catalyst beds at elevated temperatures. Such processes include dehydrogenation and aromatization. Central to the economic operation of these processes are sustained catalyst activity and efficient heat transfer.
Typical aromatization catalysts undergo both temporary and permanent loss of catalytic activity. Temporary loss of activity results from, among other factors, the accumulation of coke which blocks the catalyst pores. Both temporary and permanent loss of activity results from physical degradation or exposure to certain catalyst poisons. Temporary catalyst poisons include organic nitrogen compounds which deactivate the catalyst while they are present but are easily removed by oxidative regeneration.
In previous designs, essentially all process coke formed and was deposited on the catalyst in the aromatization zone, the very point in the process where maximum catalytic activity would be most advantageous. Thus, the aromatization process could be made more efficient, if a significant portion of coke production could be segregated from the aromatization reaction.
By blocking the catalyst pores, coke prevents the reactants from contacting the active sites of the catalyst. Coke appears to form from several different sources. A portion of the coke accumulation is attributable to thermal degradation of impurities and other easily cracked compounds in the feed. Additional coke is formed by catalytic cracking side reactions occurring concurrently with the aromatization reactions. Impurities in the feed such as oxygenates, of which glycol and furfural are examples, readily degrade to form coke upon contact with hot catalyst. To restore catalytic activity lost due to coke accumulation, the catalyst is oxidatively regenerated. During oxidative regeneration, coke burns off the catalyst as it is exposed to an oxygen-containing gas stream at elevated temperature, thereby restoring catalytic activity.
Unfortunately, however, the very process which remedies temporary deactivation causes a gradual permanent deactivation. As the catalyst is exposed to water, a regeneration by-product, at high catalyst regeneration temperatures, the crystalline structure undergoes a physical degradation commonly referred to as steam deactivation. The rate of steam deactivation is an integral function of temperature and water partial pressure. Thus, a reduction in the regeneration temperature while maintaining the desired regeneration combustion rate would be beneficial.
Heat transfer efficiency is a critical factor in the economic operation of a fluidized-bed aromatization process. Catalytic aromatization of paraffins is typically conducted at about 650.degree. C. (1200.degree. F.). Unfortunately, typical feeds can be heated in a process furnace to temperatures not greater than a few hundred degrees Farenheit lower than the aromatization or dehydrogenation temperature. At higher preheat temperatures, typical feeds crack to form coke on the heater tubes and transfer lines in addition to cracked products such as methane. The deposition of coke inside heater tubes and transfer lines can cause serious operational problems. On the other hand, the feedstock may easily be heated by direct contact with hot catalyst to temperatures about 50.degree.-200.degree. F. lower than aromatization reactor temperature without operational problems.